This invention relates generally to protecting the contacts of an electrical switch from a current surge condition, and more particularly relates to a switch which is protected against the welding or fusing together of its contacts in the event of a current surge condition.
Transient current surges and/or voltage spikes in electrical power distribution systems can occur as a result of lightning storms or other abnormal conditions in an electrical power distribution system. These surges or transients can disrupt or permanently damage electrical devices connected to the power distribution system. It is well recognized, for example, that computers are quite sensitive to power surges or transients, and various power conditioning devices or filters are available commercially to protect computers and other sensitive electronic equipment from such abnormal power conditions.
It has been overlooked, however, by many people that electrical devices which rely upon switches or relays, especially minaturized switches or relays, are also quite susceptible to disruption or damage by abnormal current surges or transients. These surges or transients can cause the contacts of the switch or relay to become permanently fused together.
The fusing of switch contacts due to current surges is a significant problem, especially in burglar alarm systems, which utilize many such switches to monitor the opening of windows and doors and to monitor the activating of motion detectors, smoke/fire detectors, and other protective monitoring devices. Once the contacts of one of these switches are fused together, the switch will not thereafter respond to the respective door opening, window opening, or other activating condition. This is troublesome in a burglar alarm system because the fused switch does not give the proper indication of its status to the user. Moreover, the existence of a fused switch is not readily apparent to the user since the burglar alarm system appears to be operating in the normal manner.
For example, a typical burglar alarm switch uses a tiny magnetic contact reed switch for monitoring the doors and windows. The magnetic contact reed switch normally operates by its contacts being maintained in a closed position when an external permanent magnet is located nearby. When the permanent magnet is moved away from the switch, as would occur when the door or window is opened, the contacts open so as to cause an open-circuit condition in the monitoring loop of the burglar alarm system. If the burglar alarm system is in the operational mode so that a monitoring control station recognizes this open-circuit condition, an alarm condition occurs.
However, a transient current surge condition, such as may occur during a lightning storm, may disrupt this normal operation mode. When a current surge occurs on the monitoring loop due to a transient on the power input line, this current surge may fuse the contacts of the magnetic contact reed switch together. Consequently, after such fusing, when a door or window is opened, the contacts remain in a closed position instead of being in an open-circuit condition. The monitoring control station recognizes the monitoring loop as still being completed or, in other words, that the door or window is still closed when it actually is not.
Unless each switch device in the monitoring system is individually tested, the user will not ordinarily know that any of the switches are fused, since they are contained in a compact housing and cannot be easily inspected. As a result, the user will believe that the burglar alarm system is operating properly, when in fact, those windows or doors having fused switch contacts will no longer be protected by the alarm system. Similar problems are presented with other types of monitoring devices which use small contact switches, or magnetic contact reed relays such as smoke/fire detectors, motion detectors, and the like.
With the foregoing in mind, it is a primary object of the present invention to provide an improved electrical switch apparatus in which the switch contacts thereof are protected from fusing caused by current surges.
It is a further and more specific object of the present invention to provide an improved electrical switch apparatus which will indicate when the switch is no longer operable so as not to give the user a false sense of security about the operability of the switch.